Well you can just set everything to be one sided and look for holes or, if your object is very complicated, try this:
- select all surfaces and make sure they are 1-sided (press '1S').
Press control+c and then control+v to copy and paste a duplicate in the same position.
use Surface->flip-normals to reverse the new surfaces
set a bright color that you are not using e.g. yellow.
Now, if you can see yellow surfaces, those are the areas where your surface normals probably need flipping. You'll need to remove the duplicate of course.